Category Archives: The Professor’s Thoughts

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How do CNC Machine Tools Move?

Only 3 possible motions…

There are really only three ways a standard CNC machine tool moves today: linear motion, arc motion, and what is called rapid motion. To understand these types of motion we should talk a little bit about how machines are made and how they are program. To do this we will first look at the construction of a 3 axis vertical machining center, or milling machine.

For this discussion we will start with the z-axis and the spindle since it is the rotation of the tool by the spindle that makes all of the cutting happen. A typical setup for vertical machining center is to have the tool mounted in a spindle which is aligned with the Z axis of the machine. This spindle is typically attached to a mechanism or “way” that allows vertical motion along the z-axis of the machine. Note that misalignment between the spindle and the z-axis way can lead to error in the finished part.

The typical z-axis drive will be a servo motor connected to a ball screw. Ball screws are really quite fascinating in how they work but for the purpose of this discussion you can imagine it simply like a nut on a bolt or screw. If you remove the threads from each end of the screw and mount the smooth parts in fixed bearing blocks with a nut in between them. The nut will move up and down along the screw as you turn it as long as you don’t allow the nut to rotate. If the spindle is then attached the nut you can turn the screw and make the spindle go up and down. To keep the nut with the spindle attached from rotating you need to attach it to something that is fixed rotationally simply welding it to the back of the machine won’t work. Instead it is attached to a bracket that is intended to slide along a guide rail. This guide rail is typically called the way. A typical axis might have two ways with a ball screw between them.

The x-axis is designed and constructed similarly to the z-axis and is installed orthogonally to the z-axis typically moving from right to left from the operator’s perspective. A y-axis can then be mounted orthogonally to the z and x-axes forming a right hand coordinate system.
Each axis is typically driven by a servo motor. Servo motor control systems, at a minimum, require feedback about position to operate. The typical method for getting this information is to attach a rotary encoder between the servo and the ball screw. This allows the controller to know “where” the spindle or x and y tables “are” at any given time as long as it hasn’t lost track of the encoder pulses which measure rotation of the motors. Because of this machine tools of this construction do not know where they are when they are turned on and will have some type of homing sequence that must be run before running any programs. The homing sequence will move each axis on at a time to the end of its travel where a switch is installed. They will drive the axis slowly across the switch noting the encoder position at the instant the switch makes contact and defining the “zero” or “home” position for this instance of operation.

As to programming: the typical machine tool today is programmed with a text based language that is sort of standardized, and goes by several names. I’ve heard NC-code, G-code, G&M-code, and simply code, or machine code. In this text I will try to stick to the terms “code,” and “g-code.” Several standard have been written over the years but as the technology has been developing so quickly the standards didn’t cover all situations individual machine tool makers wanted to address, so there are a core of “codes” common to most machines but when implementing common macros and canned cycles there can be great variation from one machine tool maker to another.
Why call it G-code or G&M-Code? The answer is simple most lines of code start with either the letter G or M followed by a string of numbers or letters that mean something to the machine tool controller. A typical command for example would be something like “G00 X1.4 Y3.5 Z4.0 F96.” This command tells the machine tool to move the axes as fast as possible (G00) to the Cartesian coordinates X=1.4 Y=3.5 Z=4.0 at the rate of 96 inches per minute (assuming the “English” system of units is being used.) Another common command might be “M03 S12000”. This tells the machine tool controller to start the spindle I the clockwise direction (M03) at 12,000 RPM (S12000). It is important to note that It doesn’t matter what the position of the machine tool axes are when the first command is received the machine tool moves as fast as it can from wherever it is to the programmed end position. Another thing to note is that the S12000 can be omitted if the last spindle command was to spin at 12,000 RPM as Spindle speed is a modal command and will stay in memory until it is changed.

Future posts will include a detailed discussion of Machine tool programming and G and M codes. The above paragraphs are enough information to continue the initial discussion on machine tool motion. As stated above there are only three ways a typical modern CNC tool can be told to move.

  • G01 – linear motion
  • G02 or G03 – arc motion (constant radius)
  • G00 – rapid motion

Let’s look at each in turn

G01 pronounced “gee zero one”

The G01 command is given when the programmer wants the tool to move in a straight line relative to the workpiece at a specified feed rate. This command is typically used when cutting is happen or imminent. The feed rate can be specified on the G01 line or previously in the program as feed like speed is a modal command. No starting point is specified. The machine tool commands the servo motors to move from the current position to the end point specified at the feed rate specified accelerating from the current location and decelerating to stop at the end point.
If motion in only one plane or along one line is desired it is possible to omit endpoint coordinates for the axis or axes that will not need to move. For example if the tool is at X=0.1, Y=3.0, Z=0.01 when the G01 command is given and the desired end point for the move is X=0.1, Y=7.0, Z=0.01, the command G01 Y7.0 is equivalent to the command G01 X0.1 Y7.0 Z0.01 and preferable as it is easy for someone looking at the program to recognize that only motion in the Y direction is called for at this point in the program.

G02 pronounced “gee zero two”

G02 like G01 is typically used when the programmer intends for the tool to be in contact with the workpiece, i.e. when cutting is happening, or when the tool is entering or exiting the workpiece material these entry and exit moves are typically called lead in and lead out or entry and exit respectively. G02 commands the machine to move from its current location to a specified endpoint like G01, but instead of interpolating a straight line the servo motor controllers are commanded to interpolate a constant radius arc motion. To this end the programmer must specify the desired end point of the motion, and a radius to define the arc. There are a couple of common ways to define the radius those will be discussed in future posts on programming. The command “G02 X1. Y1. R1.” tells the controller to move the tool from where ever it is to the point X=1.0 Y=1.0 Z= (whatever it is at the start) along an arc with a 1 inch radius (again assuming the English system of units.)

It is important to note that if the straight-line distance from the current position to the commanded end point is less than 2 times the radius the command will fail generating an error code on the machine controller.

It is possible to make arcs in the x/y plane, the x/z plane, or the y/z plane but you need to tell the machine controller which plane you want to arc in. There will be more discussion on this in posts on programming.

G03 pronounced “gee zero three”

G03 acts just like G02 except the arc is counter clockwise.

G00 pronounced “gee zero zero”

The G00 command is typically used for positioning the tool and is not usually used when the tool is intended to be in contact with the workpiece. It is used for moves to and from the tool change location and moves or “links” from the end of one cutting operation to the start of another.
G00 motion is called rapid motion and it’s goal is typically to waste as little time as possible performing non value-added motion. For setup operations and proving out (testing) programs most machine tools will have a rapid override command that allows the operator to decrease the full speed rapid to some percentage of the machines full speed capability. Operators should know something about the machine when selecting rapid overrides; I regularly use machines that move at about 600 inches per minute at full speed rapid, and another that moves at 2000 inches per minute. Twenty five percent rapid 2000 in/min machine is almost equal to full speed rapid on a 600 in/min machine.

The other thing operators and programmers should be aware of is the fact that many engineers are lazy and the easiest way to implement rapid motion in the machine controller does not allow the tool to move in a straight line from the current point to the programmed end point. The path the tool will take is predictable, but not a straight line! (By “easiest” I mean the way that requires the least math and thus the least processing resources)

dogleg rapid

The easiest way to implement rapid motion is to command each axis independently, but simultaneously. For the command “G00 X10.0 Y4.0 Z-3.5” the controller looks at the end point for x and compares it with the current x position if they are not the same it starts the x-axis motor spinning at full speed in the correct direction continually checking if it has reached the correct value. When the current position equals the desired end position the x-axis motor is stopped. At the same time the controller is moving the y and z axis servo motors checking their current positions and the commanded end positions stopping each servo when its axis has reached the commanded position.

Many of the most modern controllers have options that will disable dogleg rapid but it is best for programmers and operators to know that the machine they are using today probably moves this way.

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What’s the cost of changing a lightbulb

I went to school with a lot of people that were good at math. Walking home to my apartment one afternoon of my senior year I noticed a for sale sign in front of a house. It was a three family house like almost all of the others in the neighborhood. Very similar, in fact, to the one I was renting an apartment in at that time. When I got home I called the number on the sign just to see how much a house like that cost.

Being good at math myself it was easy to understand that rent from one of the apartments was probably enough to pay the mortgage, leaving one to live in and another to live off. I didn’t buy that house but it wasn’t long before I was on the path to becoming a slum lord in a college neighborhood. Early in my real estate career I didn’t have a lot of disposable income and spent a lot of time shopping in discount stores following the save a buck style of accounting popular with slumlords and accounting departments at companies around the world. A box of six light bulbs for a buck, you bet that was in my cart.

After 15 years and at least 100 tenants I’ve divested myself of all of my residential rental properties and would never consider buying a cheap light bulb again. Why you ask. What’s the cost of changing that cheap light bulb? I can tell you from experience it’s a lot more that the $0.17 I paid for it at the discount store.

The cost of changing a light bulb is the cost of the interruption of dinner with friends and family when you get a phone call from an angry tenant who cannot see to put their key in the lock. They are especially angry because the light has been out for weeks and you haven’t done anything about it. There is no point reminding them that you don’t go visit them every evening and if they don’t tell you they light is out you don’t know.

This interruption that ruins your mental peace is not the only cost though. There is additional cost. There is the cost of the trip to your barn to get a ladder and put it in your truck. There is the cost of the cost of going to the home improvement store to get light bulbs because you can never find the ones you got at the discount store. There is the cost of leaning the ladder against the railing of the stairs to reach the offending non-functioning bulb. And of course, there is the cost of sweeping up the glass from the bulb you dropped from the top of the unstable ladder. Not to mention the cost of putting the ladder away and figuring out where you put the original discount bulbs so you can put away the ones that are left over from this job.

At the peak of my rental management business it was really a sideline business for me as I was spending most of my time traveling and consulting for companies like General Motors and Goodyear not to mention the federal court system. For the type of work I was doing I could bill upwards of $2000 per day. Even if you divide the day into 24 hours that’s still almost $85 per hour ($250 per hour if you manage to work only 8 hours.)

Depending on how you do the math it cost me between $170 and $500 to change a lightbulb during those years. The only comparison shopping I do when looking at light bulbs is to find the ones that last the longest.

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My 6th year as an undergrad made me who I am today.

A couple of years ago I attended a graduation party for one of the most amazing students I’ve ever had the privilege to work with. He was wearing a t-shirt that said:

“College – the best 7 years of my life!”

I kidded him about it because I knew that he had not finished in the typical(?) four years and he admitted that it had indeed been seven. Not only that someone gave him the shirt his freshman year.

It made me think back to the end of August 1994 when I was about to begin my 6th year enrolled as an undergraduate engineering student.

I had plenty of excuses and rationalizations for the fact that I wasn’t done but none of them mattered what mattered was that I had no intention of asking my parents for money to pay for this year – a year that should not be – and that I had used up all of the money I made during year four to pay for year five. (I took year four “off” and worked full time.)

With almost no cash and a full course load for two more semesters staring me in the face I signed up for the payment plan offered at the bursar’s office and got a full time job driving a forklift in a warehouse attached to a plastic injection molding company. This was my first experience working directly for a manufacturing company and it was an eye opening experience.

It was also a tiring experience you see I was still taking a full course load and was working second shift five days a week plus taking all the weekend overtime they would give me. Amazingly enough I did not fail all of the classes I took that semester, but I failed enough of them to know that I needed a better plan for the spring.

I still use some of the credit cards I applied for that winter and finished the spring semester passing 12 classes with all As and Bs and almost $35,000 in credit card debit that I was very adept at moving from card to card in rotation.

In one sense, looking back, I can see that I manufactured both my failure that first semester and my success the second. During the first semester I misused my resources my time and my focus. One of the only classes I passed that semester was a class I now teach at that same university and I often wonder if the C that I got was actually a gift from the instructor.

In the spring of 1995 on the other hand success was my only acceptable result and I was able to focus on the things required to pass my classes and manufacture my success.